In general terms, electric subsea installations and devices usually demand high standards regarding durability, long-term functionality and independence during operation. Electric subsea installations that need to be cooled during operation, such as subsea converters, require an autonomous and durable cooling of its components. It is known to use a dielectric liquid of low compressibility such as for example mineral oil as a cooling fluid. The dielectric fluid could also be composed of natural or synthetic esters. In general terms, the dielectric fluid is used to provide a pressure compensated environment, and additionally functions as an electric insulation medium of electric components, such as capacitor units, placed in the electric installation. The tanks of power-electronic subsea equipment, such as subsea converters, are thus typically filled with oil, which acts as a combined electric insulation and cooling medium. The oil receives heat from the internal converter components and transfers it to the sea-water through the tank wall or through a heat exchanger.
In some cases the tank is provided with a pressure compensation system, so that the internal pressure is close or equal to the external pressure. Arrangements comprising such pressure compensation systems will henceforth be called pressure compensated arrangements. The provision of a pressure compensation system imposes significantly less stress on the tank walls in comparison to tanks (such as being part of a subsea electrical system) without pressure compensation systems. For example, the pressure at 3000 meters depth is 300 bar.
Commonly, a power converter requires several electrical connections to a transformer. Known subsea power systems typically position the power converter and the transformer in separate tanks with separate pressure compensation systems, and with wet connections for the electrical coupling.
FIG. 1 schematically illustrates such a known pressure compensated subsea electrical system 1a. The pressure compensated subsea electrical system 1a comprises a first tank comprising a transformer 3 and a second tank comprising a power converter 4. The tanks are joined by a connection. Each tank is filled with a dielectric fluid 12 and has its own separate pressure compensation system 2a, 2b. 
WO 2008/055515 (see, especially FIG. 3 therein) describes a converter and a transformer, both located within one liquid-tight housing. According to WO 2008/055515 the converter is located within a further liquid-tight housing and there is consequently no fluid communication between them.
EP2579438 (see, especially FIG. 6 therein) discloses a converter and a transformer but does not mention fluid communication.
Known from prior art (such as in WO 2008/055515) is also a pressure compensated subsea electrical system where a liquid-filled converter tank is placed inside a main vessel which also houses the transformer. FIG. 2 schematically illustrates such a known pressure compensated subsea electrical system 1b. The subsea electrical system 1b thus comprises a first tank, filled with a dielectric fluid 12, comprising a transformer 3 and having a pressure compensation system 2a. The first tank in turn further comprises a second tank. The second tank, also filled with a dielectric fluid 12, comprises a power converter 4 and has a pressure compensation system 2b. 
In general terms, the power converter has high thermal losses but requires low temperatures in order to operate efficiently. The tank wall surface is in general not sufficient to achieve the required cooling. The surface can be extended by using tank corrugations, cooling fins, or an external heat exchanger, but this increases the cost and weight of the pressure compensated subsea electrical system. On the other hand, the transformer is less sensitive to high temperatures.
EP 2 717401 A1 relates to a subsea electrical power system. The subsea electrical power system includes a first subsea electric device having a first subsea enclosure and a second subsea electric device having a second subsea enclosure. The first subsea electric device and the second subsea electric device are mounted on a common frame. A duct is provided between the first subsea enclosure and the second subsea enclosure.
In view of the above there is still a need for an improved pressure compensated subsea electrical system comprising a transformer and a power converter.